The 2007 Microsoft Office suite Service Pack 1 delivers important customer-requested stability and performance improvements, while incorporating further enhancements to user security. This service pack also includes all of the updates released for the 2007 Office suite prior to December of 2007. You can get a more complete description of SP1, including a list of issues that were fixed, in the Microsoft Knowledge Base article 936982:
http://support.microsoft.com/kb/936982 (currently gives an error).